The present invention relates to semiconductor lasers and, more particularly, to a semiconductor laser used as a light source for information processing or optical communication.
FIG. 9(a) is a cross-sectional view of a ridge type semiconductor laser disclosed by J. Hashimoto et.al. in IEEE Journal of Quantum Electron, Vol.33, pp.66-77, 1997. This laser comprises a p side electrode 1, an insulating film 2, a p type GaAs contact layer 3, a p type GaInP upper cladding layer 4 having a stripe-shaped ridge extending in the resonator length direction, a ridge side undoped GaInAsP second guide layer 5, a ridge side undoped GaAs first guide layer 6, an active layer 7, a substrate side undoped GaAs first guide layer 8 having the same composition ratio and thickness as those of the ridge side first guide layer 6, a substrate side undoped GaInAsP second guide layer 9 having the same composition ratio and thickness as those of the ridge side second guide layer 5, an n type GaInP lower cladding layer 10 having the same composition ratio as that of the upper cladding layer 4 and the same thickness as that of the ridge of the cladding layer 4, an n type GaAs buffer layer 11, an n type GaAs substrate 12, and an n side electrode 13. FIG. 9(b) is a graph showing the refractive index profile of the semiconductor laser in the direction perpendicular to the surface of the substrate 12. In FIG. 9(a), xe2x80x9czxe2x80x9d shows the resonator length direction, xe2x80x9cxxe2x80x9d shows the direction perpendicular to the surface of the substrate 12 (hereinafter referred to as xe2x80x9cthickness directionxe2x80x9d), and xe2x80x9cyxe2x80x9d shows the direction perpendicular to both of the resonator length direction z and the thickness direction x (hereinafter referred to as xe2x80x9cwidth directionxe2x80x9d).
A description is given of the operation of the semiconductor laser. Holes and electrons are injected through the upper cladding layer 4 and the lower cladding layer 10 into the active layer 7, respectively, and recombine to generate light. The light so generated is propagated along the resonator length direction z while being influenced by the refractive indices in the thickness direction x and the width direction y, and it is amplified while being reflected between the facets of the laser, resulting in laser oscillation.
In this prior art semiconductor laser, the refractive index distribution in the thickness direction x is symmetrical about the active layer 7, until reaching the upper cladding layer 4 and the lower cladding layer 10 which are disposed on and beneath the active layer 7, respectively. That is, as shown in FIG. 9(b), the ridge side first guide layer 6, the ridge side second guide layer 5, and the upper cladding layer 4 have the same refractive indices and the same thicknesses as those of the substrate side first guide layer 8, the substrate side second guide layer 9, and the lower cladding layer 10, respectively.
As described above, in the prior art ridge type semiconductor laser, since the refractive index distribution in the thickness direction x is symmetrical about the active layer 7, the propagated light is distributed almost symmetrically about the active layer 7 in the thickness direction x. However, a refractive index difference is present in the width direction y because of the ridge of the upper cladding layer 4 when the propagated light is distributed almost symmetrically about the active layer 7 as described above, the influence of the refractive index in the width direction y on the propagated light at the ridge becomes significant, whereby a higher mode of oscillation is permitted. As a result, kinks occur due to mode competition during low-power output operation, and the output power cannot be increased in practical use.
It is an object of the present invention to provide a ridge type semiconductor laser that can shift the level at which a higher mode occurs, toward the higher power level.
Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a first aspect of the present invention, there is provided a semiconductor laser including a semiconductor substrate of a first conductivity type and having a front surface; a first semiconductor layer disposed on the front surface of the semiconductor substrate and having a refractive index that increases with distance from the semiconductor substrate; an active layer disposed on the first semiconductor layer; and a second semiconductor layer disposed on the active layer, having a refractive index that decreases with distance from the active layer, and having a ridge; wherein the refractive index distribution between the ridge and the substrate is asymmetrical about the active layer so that the center of the light intensity distribution shifts from the active layer toward the substrate, in the direction perpendicular to the front surface of the substrate. Therefore, more light is distributed to the substrate side than the ridge side, and the influence of the refractive index distribution in the width direction on propagated light is reduced, which distribution occurs due to a difference in refractive indices between the ridge of the second semiconductor layer and portions of the layer outside the ridge, thereby avoiding occurrence of a higher mode that causes kinks. As a result, the light output level at which kinks occur is increased, providing a ridge type semiconductor laser capable of high-power output operation in the practical use.
According to a second aspect of the present invention, in the above-mentioned semiconductor laser, the first semiconductor layer comprises a lower cladding layer of the first conductivity type and having a refractive index, and a substrate side guide layer disposed on the lower cladding layer and having a thickness and a refractive index; the second semiconductor layer comprises a ridge side guide layer having a thickness and a refractive index, and an upper cladding layer of a second conductivity type, opposite the first conductivity type, disposed on the ridge side guide layer and having a refractive index; and at least one of the thickness and the refractive index of the substrate side guide layer is larger than that of the ridge side guide layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use.
According to a third aspect of the present invention, in the above-mentioned semiconductor laser, the refractive index of the lower cladding layer is larger than that of the upper cladding layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use. In addition, the expansion of the far field pattern is reduced, whereby the aspect ratio of light is reduced.
According to a fourth aspect of the present invention, in the above-mentioned semiconductor laser, the first semiconductor layer includes a lower cladding layer of the first conductivity type and has a thickness and a refractive index; the second semiconductor layer includes an upper cladding layer of a second conductivity type, opposite the first conductivity type, and has a thickness and a refractive index; and at least one of the thickness and the refractive index of the lower cladding layer is larger than that of the upper cladding layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use. Especially when the refractive index of the lower cladding layer is increased, the expansion of the far field pattern is reduced, whereby the aspect ratio of light is reduced.
According to a fifth aspect of the present invention, in the above-mentioned semiconductor laser, the refractive index of the lower cladding layer continuously increases toward the active layer; and the refractive index of the upper cladding layer continuously decreases with distance from the active layer. Therefore, more light is distributed to the substrate side, and the light output level at which kinks occur is increased, resulting in a ridge type semiconductor laser capable of high-power output operation in the practical use.